Gas tank overpressure after sitting in the sun


Asked by Jun 21, 2016 at 09:11 PM about the 1964 Chevrolet Malibu

Question type: General

I have a 1964 Malibu SS. Not show room quality but I like it. It drives good
and in fair condition.This weekend I filled it up with gas which I don't
normally do just to see how much it would hold.It stopped at about 17
gallons. After it stayed in the driveway for a couple of hours I noticed
something dripping right behind the back tire. It was gasoline dripping from
the frame right next to the gas tank.I went around to the license plate (thats
where the gas cap is) and when I opened the gas cap, about
a quart of gas shot out. It was a very hot day. I siphoned two gallons out
just so that wouldnt happen again.The dripping stopped also. Periodically
throughout the rest of the day I would open the gas cap and relieve
the pressure. I assumed the heat of the day was causing pressure to build
up in the tank.

I checked today and no more dripping. Now, do I have a problem with the
gas cap or the tank? How does the tank normally relieve pressure? Could I
have a hole in the tank and if so where can I take it?

3 Answers

Kelly Mayo

no vented...cleanout the vent tubes

Bob Beaman

Gasoline is stored in underground tanks at about 60 degrees F. If you crammed the tank full and it was hot outside the fuel likely expanded, due to the heat, overflowing the fuel tank. Many of the older cars had vented gas caps, but no sure about yours.

Bob Beaman

Fuel when heated will also build pressure to its vapor pressure. If you tank is unvented the buildup in pressure pushed gasoline out of the tank when you took off the gas cap. Do some reading on vapor pressure for gasoline.

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